Power Inverters and Converters

If you are intent on taking your band or set to the streets or you feel like setting up an out door gig where power is an issue, then there are a few good ways of getting round the problem. Generators and battery operated systems provide good alternatives to consumer mains, but if you haven’t got the cash to splash out on a decent generator you may well consider opting for the cheaper and noiseless battery, charger and inverter set up.

Here we learn the differences between types of dc – ac inverter, how they work and how to select the right inverter for your rig.

Sine Wave and Modified Sine Wave Inverters

DC – AC Sine wave inverters convert the DC (direct current) supply from a power source such as a car or deep cycle battery, into an AC (alternating current) supply which can be used to power regular household equipment such as fx racks, amplifiers, stereo’s, tv’s, hairdryers, microwaves and computers etc.

300 w Inverter

There are basically two types of sine wave inverter.

Pure or True Sine Wave Inverters

A pure or true sine wave inverter converts the dc supply into a near perfect or pure sine wave, replicating the supply attained from a domestic ac power source such as a plug socket. The sine wave has very little harmonic distortion resulting in a very ‘clean’ supply and makes it ideal for running electronic systems such as computers, digital fx racks and other sensitive equipment without causing problems or noise. Things like mains battery chargers also run better on pure sine wave converters.

Ideal for all applications, the pure sine wave inverter is a must for anyone needing to convert power from a dc source to a universally useable ac supply. Unfortunately they are very expensive compared to the modified alternative.

Modified (Quasi) Sine Wave Inverters

Modified sine wave inverters are a much cheaper and somewhat rougher alternative to the pure SWI. Instead of the output being a pure sine wave, the cheaper circuitry in the MSI outputs a rough sine wave. This means equipment with circuitry that relies on the smooth oscillation of a true sine wave, like dimmer switches, PC power supplies, variable speed motors and scientific equipment like oscilloscopes etc. may not work properly or as efficiently as they would otherwise.

Comparison of Sine, Square and Modified Sine Waves

The cheapest power supplies generate square waves, which as you can see from the diagram above doesn’t really follow the arc of the sine wave to any degree. A more expensive power supply producing a modified sine wave provides a more closely matched signal to the pure sine wave, but is still not ideal.

Drills and dimmer switches produce a variable output depending on the position they are in, so it’s a gamble as to how well they will perform using a modified wave generator. You may get away with it, or you could well experience problems with reliability, noise and motor irregularity.

3000 w Inverter

As for running musical equipment such as fx racks, keyboards, amps and guitars etc. with the modified wave inverter, the advice is generally the same. Depending on your set up, and how robust your equipment is you may find you get away with it, and your equipment runs fine, but you could experience noise, equipment buzz, overheating and reliability issues. It could also affect the life span of some of your more delicate gear.

A better quality supply producing a true sine wave will certainly run 99% of your digital fx equipment, amps, synths and laptops etc. with no hassle at all, just as you would expect if you were plugging in at home.

The best advise if you are unsure as whether to take the risk and go for a cheaper MSI would be to test it out before you buy. Any respectable dealer will allow you to test the inverter on your set up and give you the opportunity to return it, if it’s not right for your system.

Choosing the Correct Inverter Power Rating

Choosing the right inverter for your set up is vital.

The efficiency of an inverter varies greatly depending on the amount of power being drawn through it. It can range from around 90% when being used at it’s full rating, to around 50% when being used with light loads. In general an inverter is at its most efficient when being used at around 1/3 to 3/4 of it’s full rating. When used at optimum levels an efficiency of around 95% is attainable.

Quick Definition

A ‘load’ is generally what ever is connected to the output of the circuit.

Inverters with Resistive and Inductive Loads

Another factor to be taken into consideration is what you are running with your inverter.

Most inverter efficiencies are rated using a resistive load.
A resistive load is generally when current is converted into something else along the lines of a lighting or heating system. So if you were powering a resistive system such as a bunch of floodlights for your gig, then the efficiency percentage given by the manufacturers should be reasonably accurate. However, if you are using the inverter to power an inductive load, ie. something that uses magnetic fields such as motors, solenoids, compressors, pumps or relays etc. then you have to take into consideration the way the motors efficiency works with the wave that is powering it.

An inductive load such as a motor works most efficiently when powered by a true sine wave, but looses a great deal of its efficiency when powered using a modified sine wave. A motor may easily use 10 – 20% more power than it would otherwise when powered with a less than perfect source. Bear this in mind when you are doing your calculations and think about the inefficiencies of everything you are intent on powering in the circuit.

To cut a long story short, a true sine wave inverter is best for every occasion, but not totally essential if you haven’t got much cash and can get away with it.

Inverter Ratings

To choose the right inverter, you need to know it’s 3 ratings and the ratings of the equipment you intend to power.

1. Surge Rating – Starting Load – or Peak Load . This is how much the inverter can handle for literally a few seconds while it deals with power spikes caused by the switching on of equipment such as amps, fans, motors, tv’s etc.

2. Continuous Rating – Continuous Load – This is the load the inverter can handle for as long as it likes. Generally, this is what the inverter’s advertised rating would be.

3. Limited time rating. This is how much power the inverter can handle for short periods of time when an excess load is placed on the system. This time period can be anything from a couple of minutes up to ten or twenty.

Obviously the peak and continuous power consumption levels drawn by the equipment you are using with your inverter i.e amps, tv’s lights etc. should in total be less than your inverters capabilities. You will normally find power ratings marked on the equipment or in the instructions.

If you are worried about whether or not your equipment, be it a light, amp, compressor or fridge etc. has a starting load to worry about, most inverters have a maximum peak load rating of 3 or more times their continuous power rating. This should probably do you in most cases but check your inverter to make sure it does. Also check the specs of the equipment you are running. Some types of machinery require starting loads many more times that of it’s normal running load.

Inverter Calculations

Here’s a rough example of an inverter rating calculation. Obviously you can add as many amps, mics mixers, monitors and a P.A to match your needs. I’ve just thrown in some equipment to give you an idea of how to proceed with your own set up.

Say you wanted to do a small set in town with a couple of mates with combo amps and an unamplified drum kit.

You might have a guitar fx unit, bass fx, a keyboard, a 30 watt combo amp, a 25 watt combo amp, a 50 watt bass amp, a reverb rack and couple of mics fed into the combo’s. (Don’t write in correcting the set up, it’s just an example)

We deal with rms values in all instances. As an amplifiers rating is generally stated in watts rms and standard Ac electricity supply is 220 volts rms (in the UK).

Work out the individual power consumption of all your fx units, amps, mixers, lights and anything else you are going to use. The power consumption of each unit should be written on the side or back of the unit or will be stated somewhere in the specification sheet in the instruction manual. If not, you can use the equation below to work it out.

For obvious reasons, I would always advise to buy equipment that can cope with work loads larger than what you initially need as your demands are bound to increase as your needs expand. Also you might want to take into consideration other losses that are not so obvious such as converting ac power back into dc for your equipment, ambient temperature, and general losses due to mechanics, age and wear and tear of all the gear involved.

I reckon an extra 25 % is a large enough excess.

That would give us… 0.25 x 166 = 41.5
166 + 41.5 = 207.5

So we would need a continuous supply of 207.5 watts RMS

Bearing in mind an inverter will have an average efficiency rating of between 85 and 95 %, we can take the value of 90% and say the 207.5 watts we need to get out of the inverter on the other side will only be 90% of the power we need to put in to achieve that.

So we have to multiply our 90% figure by some factor to work out the 100% figure we originally fed into the inverter to attain our 90% out.

90% x 1.1 = 99 %

If we multiply 207.5 watts by a factor of 1.1 we get

207.5 x 1.1 = 228.25 w

This means in theory, if we input around 228.25 watts in one end, we’ll get 207.5 out the other.

So if we went out and bought a true sine wave inverter that had a constant power rating of 250 watts we should be fine and have plenty of spare room to play with.

The Easy Way

There’s always an easier way of doing things. If you go out and buy a clamp on ammeter, you can set your equipment up in your own home and measure the actual current draw (I) on a normal domestic supply. It’s always better to have a real result rather than a calculated one. Then use P=VI to calculate the correct sized inverter for the job.

You should be able to buy a decent clip on ammeter for around £20.

Remember to check your inverter has a good peak power rating, incase you need to use it for other things like inductive loads in future.

One last thing to be aware of is that cheap or incorrectly rated connection cables between your battery and inverter can also cause noise, overheating and more efficiency problems. The distance between connections is also an issue an can create voltage drops which will affect the output of your inverter. You should refer to the manufacturer’s recommendations to find the correct cable lengths and ratings instead of using just any old cable you find in the shed.

Now you know all about inverters, their uses and power ratings, check out my other guides to help you decide what’s best for your outdoor set up.

…

For time and legal reasons I don’t answer reader’s questions on calculations or individual set up’s, but if you need to know anything about creating your own power set up or need advice on calculations, make sure you fully read through my battery guide and posts on inverters, battery calculations and battery connections.

This is great stuff, thank you. But I don’t understand why a keyboard that uses a transformer to convert AC to 12 v DC couldn’t run directly off the DC from a 12v battery instead of inverting it to AC then transforming it back into DC again??! It seems so inefficient – is it safer or something? Hope you can reply – or that Josh will put his feedback up x

Hi Sarah – Yeah, I know what you mean, but the correct power supply will have been specifically designed to produce the output that suits the needs of the circuitry in your synth and also have built in protection against overloads and current/voltage fluctuations etc. that could cause damage to the delicate instrument.

An inverter mimics the pure/clean supply from the mains that your equipment was designed for so it’ll be much happier and survive a lot longer running from the correct set up. A battery connected straight to your keyboard is very risky and could result in being a very expensive shortcut. All in all, it just depends what equipment you are trying to run. Sensitive electronics need good power supplies, other more robust things often do not.

Great information. I know someone who is using inverter to power a mobile food cart and I was trying to explain to him resistive and inductive loads and how inductive loads can be tricky. Your explanation is loads (no pun intended) better than mine so I will direct him to your info. Thanks much

I have purchased 750 VA inverter for domestic purpose along with 100AH battery. To test the back up time provided by the battery, I switched off the AC supply. The inverter supplied power for a single domestic fan rated 60W, 230 V for only 10 hours. I assume fan would consume only I = W/(V*P.F). In this case if we assume power factor 0.7 then current would be 60/(230*0.7) = 0.375 amps. Even in worst case if we assume fan would consume 0.5 amps, then back up should be for 200 hrs. Am I missing something? Please help.

I’m afraid it’s not so simple as that, there are many other variables you must take into consideration when calculating battery power. If you read my battery guide and posts on inverters and battery calculations you should be pretty clued up as to whats going wrong with your calculations.
Have a good read and try your calculations again.

Hi, I have read the battery guide, inverters, and battery calculations sections and would like to follow up on the question that “sadiq” asked above. What variable or variables as you say would diminish the actual discharge time of the battery system described in his “fan example” by a factor of 10 or more?

Granted that the battery will not completely discharge even if such a thing was desired.

It does seem from the equations and information that you have provided in your excellent articles that he should have expected much more than 10 hours of fan operation if his equipment and setup were as he described.

Thanks for some excellent information. I would like to follow up on the question asked by “sadiq” above regarding operation of a fan. What variable or variables as you suggest could reduce the amount of fan operation by a factor 10 or more as he describes.

Granted a battery will not completely discharge even if one wanted it to.

I have read the various relevant sections and don’t understand from the equations and information provided what variables could explain the dramatic reduction in operation time he described. Please explain.

Hi Guys, if you have a quick read through you will notice my posts list many variables not taken into account in your figures. Although I really love to solve people’s issues, I run a few businesses on and off line and have very little time to answer questions so I’m afraid I can’t do calculations for visitors anymore.

Often, there is a mass of information needed to solve problems and it just isn’t practical for me to keep emailing back and forth asking questions all day, so I have listed a few things below you need to take into account and hopefully once you have understood these, you may get closer to your figures.

Is it a pure or modified sine wave inverter ? There are huge differences in the way they operate and their efficiency levels.
What rating are they being used at. On top of the above, efficiencies range from 50% to 90% depending on the loads they are running in relation to their rating.
The load you are using is an inductive load, not a resistive one, again affecting efficiency.
What type of battery are you using ? Marine and deep cycle, starter ? etc. All have different levels to which they can discharge before they become useless.
What is your depth of discharge ?
What is your rate of discharge ?
Have you taken into account the Peukert effect ?
How well is the battery charged in the first place.
Has it been stored for a long time before purchase ?
Which Amp hour rating time has the battery been listed at the 8 hour or 100 hour amp rating time ?
Connection cable efficiencies, length, resistance and heat loss etc ?

As you can see, there is a lot of stuff here you need to consider to provide a reasonable accurate test. It’s all in the posts, but it’s down to you to figure out the specifications of your equipment and from there you can taylor a system to your requirements which will hopefully provide the power you need to do the job.

I hope that helps and good luck with your system.

For anyone else with similar questions, my advice is the same, read the posts, do the calculations and if you’ve done your sums right, you should get an idea of what to expect.

If all else fails, you may have dodgy equipment and should get your gear tested by professionals, just incase.

Thanks for the wonderful information. For added info, A Pure Sine Inverter is really the best among all the inverters in the market now. Though this type of inverter is much expensive than other power inverters.

I’m just about to purchase a pure sine wave 600 watt inverter to run with a car battery…. to power a Korg Keyboard, a 50 watt amp (2x input) and a small casio keyboard. Its for a weekends busking, gigging promotion and “havoc causing” across London and Brighton with my band. This page has been a massive help in my preparations. Im hoping all goes well, follow the progress on our website – http://www.amoriste.com

Kier (your a frighteningly clever bloke by the way), but have you used a car battery before…or know how long they would roughly last on the above equipment?

Great website you’ve got there and a good cause, I hope it goes well for you all, the charity too.

I’ve never used a car battery, but I do know this hill-billy banjo player who busks across the South using an old car battery wired up to some ancient amp/speaker type rig. He plays banjo over folk backing tracks for hours at a time and makes a fortune. I quizzed him on it one day but couldn’t make much sense out of him. Serves him well, but to say how long it lasts per charge, how efficient or how long his rig has survived is anyone’s guess.

sir,
i need a help .
i required a battery which will charge at high voltage & low current in a very shot period .
and out put is low voltage but for a long duration .
plz help about the composition of the battery.
i need ur help…….

Hi, I’m afraid i don’t advise readers on specific needs due to legal implications but if you read my battery guide and posts on inverters and battery calculations, by the end of that you should have a better idea of what to look for.

thanks for the amazing resource! I wish I’d found this earlier! I’ve just bought a 100ah Leisure battery and a Ring Powersource 300W (RINV300) inverter. I need to power a Boss RC50 Loop Station and a Mackie SRM350 active speaker. The RC50 works fine, but the Mackie speaker has a very loud buzz that makes it unusable! To be honest these speakers always have a buzz present, even when using mains power, but running from the inverter it is LOUD! Is this because of the quality of the inverter? I actually need the setup to run a workshop on tuesday so I’m a bit screwed if I can’t get it working!

Hi Billy, looking at the specs of the RINV300, it’s a modified inverter so it’s not good for your kit and is going to produce a bad signal anyway. Take it back and try it with a pure inverter. If it’s still not good enough, could be other things in the room adding to the problem.

Hi Billy, if like me you’ve got loads of crap in your music room, power packs leads, transformers all pluged in and producing intereference, it will have a detrimental effect on your set up. Nevertheless, pure sine wave is always the best and resembles your mains power signal at home. Quasi is a waste of time and will probably knacker your gear sooner or later.

Regarding the plastic, who cares if you had to rip it open. I do it all the time, if you are not happy with the item, for what ever reason, as long as you have not totally wrecked the box or damaged the item, a 1 pence plastic bag makes no difference. I’m always taking (and sending) things back. Shops and websites have to deal with that sort of thing, especially if they give you no choice to get the thing open. Just send it back and don’t take no for an answer.

Thanks Kier, the last thing i want to do is ruin hundreds of pounds worth of gear, so probably best to have a quality invertor anyway. As for returning, I actually bought it brand-new-second-hand on e-bay, so no chance of returning. And the plastic is those stupid heat-sealed moulded packaging types, so no box to return it in even if I could! Going to test it away from all my gear now…

There are different types of inverter batteries such as:
1.The Lead Acid Battery is made up of plates, lead, and lead oxide (various other elements are used to change density, hardness, porosity, etc.) with a 35% sulfuric acid and 65% water solution.
2.The Tubular Plate Batteries are designed extremely strong and can withstand 1400 cycles of deep discharge at 80% discharge levels in its lifetime. All the plate grids in flat plates and the spines in the tubular plates are cast in automated high-pressure machine with maximum 2.5% antimony alloy.
3.Sealed Maintenance Free Batteries, more commonly known as SMF. These batteries are fully sealed. SMF Batteries are highly recommended for Inverters. They are safe and maintenance free.

You are missing the fact that your battery is NOT 230 volts although you don’t state what it is. Presuming that the battery is 100AH at 12 volts then this is 1,200 watt hours (12+100) so your 60 watt fan should run 1200/60 hours which is 20 hours. Now figure in the inefficiences you mention and the 10 hours you find in practice is right on the money.
Note running a lead acid battery below 50% charged will kill it quickly.

I use a crompton greaves sinewave inverter and a luminous tall tubular 150 AH lead acid battery. I have found the battery chamber with the acid turning into black color. This has occured in first new battery 2 months after purchase and was replaced a new one. Again the second battery also gets two of the chambers turned into black liquid. does it mean that the battery is defective or it is the UPS charger?

im concerned about grounding, is there a large risk of electric shock or damage to equipment with a setup like this? how do you deal with grounding issues while connecting the battery and equipment together on the street?

Most inverters have “ground fault” (Earth leakage trip) protection which means they will shut down in a small fraction of a second if a leakage to ground/earth (aka you getting fried) is detected.
To be doubly safe always connect the high voltage wiring first and the battery wiring last. (Reverse order for dismantling)

I have found your website to be quite informative and helpful, thank you! I have one small math issue to point out. If you estimate that an inverter works at 90% efficiency (.9), simply dividing the wattage by .9 gives a more accurate (and mathematically sound) result then multiplying by 1.1. . In this case we are just estimating so your results will be close enough for this application, I’m bringing this this up more just to be informative math-wise.
Hopefully this example won’t be too confusing: say you need to provide 1000 watts. If an inverter provides 80% efficiency, what is the lowest wattage inverter you could get away with? Answer: 1000W/.8= 1250W. Since division and multiplication are inverse operations, we know that 80% of 1250 is exactly 1000 (.8*1250=1000) So a 1250 Watt inverter functioning at 80% will give exactly 1000 W. I’ll assume your estimated innefficiency factor would be 1.2 in this case, which would give you an answer of 1200 W…. And 80% of 1200 W is 960, which wouldn’t be enough to power your 1000 W load. So a better formula for calculating minimum inverter wattage is: (Watts you need) / (efficiency percentage)= (minimum inverter wattage). Again, your estimation would have given a fine result for a small wattage, but the bigger the numbers get, the further off you would be. Maybe I’m wrong, and if there is something I am missing in your inefficiency factor, I’d be curious to know what that is, but regardless, thanks for the information, great stuff.

Hey I’m wondering if a generous soul would like to talk to me through email to help me to understand what sort of set up I can make to power my Nord stage, a boomerang III loop pedal and a mic (not a condenser just general mic). I was thinking the marine battery option would be best with an inverter but I really don’t know where to start. I’m also in Australia which may reduce my options as its hard to find gear at times. Thank you in advance if you have the time to chat. Deanne.muir@outlook.com

Marine batteries are better than normal Lead acid car batteries BUT not by much. Get a RE (Solar panel type) for the best results without spending a fortune on NICAD (etc) batteries. Any quality SINE WAVE inverter with matching output for your amp will be fine.
Matching means 2 to 3 times the name plate power draw or your devices.

Oh and NEVER discharge any Lead Acid battery by more than 50% if you want it to last. As an example.
RE battery discharged by 50%; life cycle about 1000 times.
Car battery discharged by 80%; life cycle 50 times if lucky.